FOLYSKY（WuHan） LTD.
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FOLYSKY（WuHan） LTD.
Insulated-gate bipolar transistor (IGBT) is the most advanced power electronic device to realize electric energy conversion and control, which is widely used in electric vehicles, electric locomotives, smart grid and other fields. Silicon nitride ceramic substrate not only has the characteristics of high thermal conductivity, high electrical insulation, high mechanical strength and low expansion of ceramic, but also has the high electrical conductivity and excellent welding performance of oxygen-free copper, which is the key basic material for IGBT module packaging. In this paper, the direct copper coating process (DBC) and the active metal welding process (AMB) were used to prepare the silicon nitride ceramic copper coated plate. The similarities and differences of the two processes and the properties of the prepared silicon nitride ceramic copper coated plate were compared, and the Silicon nitride ceramic copper coated plate is expected to be widely used in the next generation power module.
1. Introduction
Insulated Gate Bipolar Transistor (IGBT) is the most advanced power electronic device for the conversion and control of electrical energy. With a series of advantages such as large input impedance, small driving power, fast switching speed, high operating frequency, low saturation voltage, large safe working area and high voltage and high current resistance, it is known as the "CPU" of modern industrial converter devices, and is widely used in strategic industries such as rail transit, aerospace, new energy vehicles, wind power generation, and national defense industry.
With the intensive introduction of a series of policies such as "Made in China 2015", "Implementation Plan for Special Action on Industrial Green Development", "Guiding Opinions on Accelerating the promotion and application of new energy Vehicles" and "UHV Planning", China's high-speed railway, urban rail transit, new energy vehicles, smart grid and wind power generation projects have become the hot spot of "green economy" in the next few years. These projects have an urgent and huge demand for high-voltage and high-power IGBT modules. Due to the high voltage and high power IGBT module, the technical threshold is higher and the difficulty is greater, especially the packaging material is required to have better heat dissipation performance, higher reliability and greater current carrying capacity. However, the domestic technical level is backward, leading to the domestic high-pressure IGBT market by Europe, the United States, Japan and other countries monopolized, high-pressure IGBT products high prices, long delivery cycle, insufficient production capacity, seriously limited the development of China's power locomotives, electric vehicles and new energy and other fields.
The heat generated by the high-voltage and high-power IGBT module is mainly transmitted to the shell through the silicon nitride ceramic coppered plate, so the silicon nitride ceramic coppered plate is an indispensable key basic material for power module packaging in the field of power electronics. It has the characteristics of high thermal conductivity, high electrical insulation, high mechanical strength and low expansion of ceramics, and has the high electrical conductivity and excellent welding performance of oxygen-free copper metal, and can etch various graphics like PCB circuit boards. Silicon nitride ceramic copper-clad plates combine the advantages of power electronics packaging materials:
1) The ceramic part has excellent thermal conductivity and pressure resistance;
2) The copper conductor part has a very high current carrying capacity;
3) High adhesion strength and reliability between metal and ceramic;
4) Easy to etch graphics to form the circuit board;
5) Excellent welding performance, suitable for aluminum wire bonding.
The property of ceramic substrate material is the determining factor of the property of ceramic copper-clad plate. At present, there are three kinds of ceramics that have been used as substrate materials for ceramic copper-clad plates, namely, alumina ceramic substrate, aluminum carbide ceramic substrate and silicon nitride ceramic substrate. Alumina ceramic substrate is the most commonly used ceramic substrate, because it has good insulation, good chemical stability, good mechanical properties and low price, but because of the relatively low thermal conductivity of alumina ceramic substrate, it is not well matched with the thermal expansion coefficient of silicon. As a high power module packaging material, the application prospect of alumina material is not optimistic.
Aluminum nitride copper clad plate has very high thermal conductivity and fast heat dissipation in terms of thermal characteristics. In terms of stress, the coefficient of thermal expansion is close to that of silicon, and the internal stress of the whole module is low, which improves the reliability of the high-pressure IGBT module. These excellent properties make aluminum nitride copper clad plate the first choice for high voltage IGBT module packaging. In this paper, the preparation of aluminum nitride ceramic copper clad plate by direct copper coating process (DBC) and active metal welding process (AMB) was studied, and the similarities and differences of the two processes and the properties of the prepared aluminum nitride ceramic copper clad plate were compared.
2. Study on the preparation of aluminum nitride ceramic copper clad plate by direct copper coating process (DBC)
The so-called DBC technology refers to a layer of copper foil directly welded on the surface of alumina or aluminum nitride ceramics at a high temperature of about 1063 ° C in oxygenated nitrogen. The basic principle is that the cop-oxygen eutectic liquid phase formed during the sintering of copper and oxygen is used to wet the surfaces of the two materials in contact with each other, that is, the copper foil surface and the ceramic surface. At the same time, the composite oxides such as CuAlO2 and Cu(AlO2)2 are also generated by the reaction with alumina to act as the solder for eutectic brazing to achieve the solid combination of copper foil and ceramics []. However, since aluminum nitride is a non-oxide ceramic, the key to coating copper foil is to form an oxide transition layer on its surface, and then to achieve the coating of AlN and Cu foil through the transition layer and Cu foil coating.
Based on the above basic theory, we systematically studied the surface oxidation, oxygen-free copper oxidation and direct copper coating of aluminum nitride ceramic, optimized the process parameters, and prepared aluminum nitride ceramic copper coating plate. No significant voids were found in the sample, especially in the core region, and the content of voids at the upper and lower interfaces was less than 3%. The sample was cut into 10mm wide strips of prefabricated incisions to test the pulling force of copper from the ceramic surface. The peeling strength of the samples was greater than 60N/cm.
The interface of ceramic and copper is closely bonded, and the structure is dense. The ceramic grain is about 1-5μm, and there is a transition layer between 8-10 μm and copper. The transition layer is compact and has a grain size of about 3-5μm, but there are discontinuous micro-cracks between the grains. The ceramic surface is dense and there are no pores. The surface particles are uneven, which may be caused by the crack spreading along the grain boundary when it is pulled apart, some particles on the copper and some particles on the ceramic.
Preparation of aluminum nitride ceramic copper-clad plate by triactive metal welding process (AMB)
Active copper welding process is a further development of DBC process technology, which uses a small amount of active elements contained in brazing metal to react with ceramics to form a reaction layer that can be wetted by liquid brazing metal, so as to realize the joint of ceramics and metals. First, the ceramic surface is printed with active metal solder, and then it is clamped with oxygen-free copper and welded at high temperature in a vacuum brazing furnace. After cladding, the substrate is made of circuit on the surface by wet etching process similar to PCB board, and finally the surface is coated to prepare a product with reliable performance. The AMB substrate is bonded by chemical reaction of ceramic and active metal solder paste at high temperature, so its bonding strength is higher and its reliability is better. However, due to the high cost of this method, the suitable solder is less, and the solder has a greater impact on the reliability of welding, only a few Japanese companies have mastered the highly reliable active metal welding technology.
Through the optimization of different solder formulations, we developed a special active solder paste system for aluminum nitride ceramic active welding. The solder paste has the characteristics of simple preparation process, excellent printing characteristics, good wettability with aluminum nitride ceramic and high bonding strength after welding.
Solder paste screen printing technology and vacuum welding technology are used to achieve good welding of aluminum nitride and copper. Through research and optimization of welding methods, the welding strength and welding interface of aluminum nitride and copper are well controlled, the interface cavity rate is less than 1%, and the welding process curve is solidified.
The AMB substrate requires good weldability and good bonding force at 250℃ during pressure welding, so its surface needs to be nickel-plated. However, after etching the pattern of AMB substrate, there are a lot of islands on the surface, it is difficult to electroplating and the thickness of the coating is not uniform, so electroless nickel plating is undoubtedly the best choice. In order to improve the uniformity of nickel plating layer, nickel plating on aluminum nitride ceramic copper-coated substrate was achieved by electroless plating Ni-P technology. Through the optimization of plating solution and plating parameters, the thickness of nickel layer could be controlled within 3-5μm, and the uniformity could be controlled within ±0.2μm. At the same time, the bonding thrust of aluminum nitride copper clad plate is greater than 1700g, which meets the application reliability requirements of high-pressure IGBT module.
The reliability of batch temperature impact of aluminum nitride copper-coated substrate is the key factor of its performance. It is required that aluminum nitride copper-coated substrate can withstand 100 temperature cycles of -40℃ ~ +150℃ after chip welding. We have carried out a series of optimization of aluminum nitride ceramic copy-covered substrate, including the design optimization of copy-covered substrate and the process optimization of copy-covered substrate, and finally fully met the requirements of temperature cycle reliability.
Through the process research, our company has independently developed two types of aluminum nitride ceramic copper coated substrate, it can be concluded that AMB process has higher reliability and better comprehensive performance than DBC process, and the aluminum nitride copper coated substrate produced by our company has been comparable to the technical indicators of related products produced by Japanese companies.
Fourth, the development direction of ceramic copper-covered substrate for high-pressure IGBT module
The appearance of the third generation semiconductor materials represented by silicon carbide and gallium nitride provides the possibility for further improvement of device performance. In view of the application requirements of high frequency, high temperature and high power of SiC/GAN-based third-generation semiconductor devices, it is necessary to develop ceramic copper-covered substrates with higher reliability, better temperature resistance and stronger current carrying capacity in order to realize high-density three-dimensional modular packaging of high-power power electronic devices. Silicon nitride ceramic substrate has a low bending strength of 2.4 times that of aluminum oxide and aluminum nitride, so it has much higher reliability than aluminum nitride and aluminum oxide, especially high strength can realize its overlay with thick copper substrate, greatly improving the thermal performance of the substrate. Compared with aluminum nitride and alumina, silicon nitride ceramic copper clad plate has obvious advantages in current carrying capacity, heat dissipation capacity, mechanical properties and reliability. At the same time, β-Si3N4 ceramics have a potential high thermal conductivity (200~320W/m•K), but their microstructure is more complex, and the scattering of phonons is large, so the thermal conductivity is low [], which limits its application as a power module substrate material. Therefore, more research is focused on how to improve the thermal conductivity of silicon nitride ceramics.
High thermal conductivity ceramics should have the following conditions:
(1) The average atomic weight is small;
(2) High atomic bonding strength;
(3) The crystal structure is relatively simple;
(4) The lattice anharmonic vibration is low.
Methods for improving the thermal conductivity of silicon nitride ceramics include:
(1) Introduction of β-Si3N4 phase seed;
(2) Selection of sintering additives;
(3) molding process and heat treatment process.
Therefore, in the field of high-power IGBT modules, silicon nitride ceramic copper clad plates are widely used in high-reliability power modules for electric vehicles in the future because of their ability to weld thicker oxygen-free copper and higher reliability. According to the material and process characteristics, the technical development direction of ceramic copper clad plate is shown. In the field of high-power power module, aluminum nitride ceramic copper clad plate is the main development direction, and in the field of high-reliability power module, silicon nitride ceramic copper clad plate is the main development direction.
With the rise of China's strategic emerging industries, power electronics technology plays an important role in important areas such as wind energy, solar energy, heat pumps, hydropower, biomass energy, green buildings, new energy equipment, electric vehicles, rail transit and other advanced manufacturing industries, and many of these areas have a market size of more than one trillion yuan in the "13th Five-Year Plan". It will certainly bring about the rapid development of power electronics technology and its industry, ushering in a significant period of development opportunities. These will create a huge demand for ceramic copper-clad plates, the key material for IGBT module packaging. Therefore, it is necessary to seize the opportunity to develop a series of ceramic copium-covered substrates to meet the needs of different fields, especially the need to speed up the development and industrialization of highly reliable aluminum nitride substrates and silicon nitride substrates, and lay the foundation for the localization of high-pressure IGBT modules in China.
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